This application claims the priority of Korean Patent Application No. 2002-009281, filed Feb. 21, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber in a high speed optical transmission system and an apparatus therefor.
2. Description of the Related Art
Recently, communication data has been rapidly increased due to an increase in a demand for Internet, and such a trend will be increased more in future. An optical transmission system with a wide bandwidth is required to accept a large capacity of data. In order to manufacture the optical transmission system, time division multiplexing (TDM) and wavelength division multiplexing (WDM) have been studied in a wider range. In TDM, as a bit rate increases, polarization mode dispersion (PMD) becomes influential.
In PMD, arbitrary birefringence is led to optical fiber on a transmission path due to internal causes such as an asymmetric optical fiber core structure and stress, and external causes like variations in an environment, such as variations in temperature, vibration, and pressure around a transmission path, thereby an optical signal by birefringence undergoes waveform distortion. The waveform distortion occurs due to a differential time delay of two polarization components perpendicular to each other commonly referred to as principal state of polarization (PSP) in transmission fiber. In this case, each PSP is transmitted without change of waveform. In a case where a time delay having a size same as the differential time delay of the two PSPs and having a reverse direction is forcibly applied to the two PSPs, the two PSPs are offset against each other, thereby distortion due to PMD is compensated. Several studies according to the principle are as below.
There is the article entitled xe2x80x9cPolarization Mode Dispersion Compensation by Phase Diversity Detectionxe2x80x9d, by B. W. Hakki, and published in IEEE Photonics Technology Letters, Vol. 9, No. 1, p 121-123, and the structure of a polarization mode dispersion (PMD) compensator 1500 that is proposed by B. W. Hakki and is referred to as prior art 1 is shown in FIG. 15. In the PMD compensator 1500, a polarization beam splitter 1530 optically separates two polarization components passing a transmission path, and a differential time delay is obtained from a mixer 1540, and a time delay having a size same as the differential time delay and having a reverse direction is electrically applied to the two polarization components, and the two polarization components are added by a combiner 1590, thereby a distorted signal is compensated. However, in the PMD compensator 1500, as a speed of a transmitted optical signal, that is, a bit rate, increases, a differential time delay should be calculated more precisely, and thus the mixer 1540 requires expensive high speed electronic devices. The physical length of the delay line 1570 is limited, resulting in restricting a compensation range, and mechanical movement is required for operation of the delay line 1570, and thus may make a bad effect on the reliability of a system.
A technique for compensating a differential time delay due to PMD by controlling an optical delay line and a polarization transformer in a Mach-Zehnder interferometer type PMD compensator by monitoring electrical spectrum is disclosed in U.S. Pat. No. 5,930,414 entitled xe2x80x9cMethod and Apparatus for Automatic Compensation of First-Order Polarization Mode Dispersion (PMD)xe2x80x9d, by Fishman et al., and published on Jul. 27, 1999. A compensator 1640 that is disclosed in U.S. Pat. No. 5,930,414 and is referred to as prior art 2 is shown in FIG. 16. In the compensator 1640, an optical signal output from a tap 1643 via a Mach-Zehnder interferometer 1642, passes through a photodetector 1646 and an amplifier 1645, and obtains an electrical power depending on a differential time delay. The control signal is applied to an automatic polarization transformer 1641 and an optical delay line 1642 using the electrical power as a feedback signal, the direction and size of PMD of the optical signal are controlled, and then the output of the optical signal is again monitored. As a result of this iterative process a compensated signal is eventually achieved. In the compensator 1640, an output signal of the amplifier 1645 passes a filter to a RF power detector included in a distortion analyzer 1644 and is integrated in a given frequency range so as to obtain an unambiguous feedback signal. However, the compensator 1640 employs a method for compensating PMD by a limited range of optical delay line, and thus, there is a limitation in a PMD compensation range, and the reliability of the system may degrade due to the use of the delay line that operates mechanically. An integrator is used to obtain an unambiguous signal, and thus an additional integration process is required. The control method of adjusting all possible polarization states with the automatic polarization transformer 1641 for each given differential time delay of the delay line 1642 may require relatively much time to obtain a final compensated signal.
In addition, the configuration of the above techniques is relatively complicated.
A technique for using only one polarization component between two principal states of polarization (PSPs) as a compensated signal is disclosed in U.S. Pat. No. 6,130,766 entitled xe2x80x9cPolarization Mode Dispersion Compensation via an Automatic Tracking of Principal State of Polarizationxe2x80x9d, by Cao et al., and published on Oct. 10, 2000. A compensator 1720 that is disclosed in U.S. Pat. No. 6,130,766 and is referred to as prior art 3 is shown in FIG. 17. In FIG. 17, an optical source 1712 at a transmission terminal 1710 is frequency modulated using a dithering input signal and a driver 1711. By controlling a polarization controller (PC) and tracking PSPs in a way that minimizes a second-order harmonic component of an interference signal between two PSPs that is detected from one output of a polarization beam splitter (PBS) 1725, axes of the two PSPs coincide with two axes of the PBS 1725, and therefore a undistorted compensation signal is obtained by selecting only one PSP between the two PSPs existing in the output of the PBS 1725. However, the above compensation method requires an additional apparatus such as a driver for frequency-modulating at a transmission terminal and additional manipulation. Moreover, a relatively complicated digital signal processing method using a DSP control unit 1722 is used. A smaller signal between the two PSPs may be selected as an output during PSP tracking, and thus, the reliability of the system cannot be guaranteed.
To solve the above problems, it is a first object of the present invention to provide a method for compensating polarization mode dispersion (PMD), in which there are no limitations on a PMD compensation range, and the reliability of a system can be guaranteed by allowing the optical power of a compensated output signal to be over half of input power, apparatus, and system therefor.
It is a second object of the present invention to provide a method for tracking principal state of polarization (PSP), in which a compensated signal is eventually achieved by iterative feedback control to minimize an electrical power at a certain specified frequency such as bit-rate frequency or its harmonics for NRZ format and twice the bit-rate frequency or its harmonics for RZ format.
It is a third object of the present invention to provide a method for compensating polarization mode dispersion (PMD), in which PMD can be compensated at a high speed by a simple optical structure and a simple signal processing method without requiring high-priced high speed electronic elements, apparatus, and system therefor.
Accordingly, to achieve the first, second, and third objects, according to one aspect of the present invention, there is provided an apparatus for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The apparatus includes a polarization controller (PC) for transforming states and directions of polarization components of an optical signal received from the optical transmission line, a polarization rotator for rotating the polarization components of the optical signal output from the PC, a polarization beam splitter (PBS) for separating two orthogonal polarization components of the optical signal output from the polarization rotator, for transmitting a first polarization component to an signal output path and transmitting a second polarization component to a monitoring path, a compensation controller for controlling the PC using the electrical spectrum of the optical signal transmitted to the monitoring path so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS, and a rotation controller for controlling the polarization rotator by comparing an optical power of the first polarization component transmitted to the signal output path with an optical power of the second polarization component transmitted to the monitoring path.
It is preferable that the optical signal transmitted to the monitoring path is detected by a photodetector to obtain its electrical spectrum.
It is also preferable that the compensation controller includes a comparing unit for comparing a presently detected electrical power filtered at the predetermined frequency satisfying the previously described principle of operation, with the previously detected one, and a feedback signal applying unit for applying a feedback signal to the PC so that the one having a smaller magnitude between the presently detected electrical signal and the previously detected electrical signal is selected as the result of comparison.
It is also preferable that the first polarization component and the second polarization component are measured by a first optical power meter and a second optical power meter, respectively.
It is also preferable that the rotation controller includes an optical power comparing unit for comparing a first optical power that is measured from the first optical power meter with a second optical power that is measured from the second optical power meter, and a rotation command applying unit for applying a 90 degree rotation command to the polarization rotator in a case where the first optical power is smaller than the second optical power as the result of comparison of the optical power comparing unit.
It is also preferable that the polarization rotator includes a faraday rotator.
In order to achieve the first, second, and third objects, according to another aspect of the present invention, there is provided an apparatus for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The apparatus includes a polarization scrambler installed at a transmission terminal, for exciting polarization of input light to optical transmission fiber with equal probability in all directions and transmitting the scrambled light to an the transmission line, a polarization controller (PC) for transforming states and directions of polarization components of an optical signal received from the optical transmission line, a polarization beam splitter (PBS) for separating two orthogonal polarization components of the optical signal output from the PC, for transmitting a first polarization component to an signal output path and transmitting a second polarization component to a monitoring path, and a compensation controller for controlling the PC using the electrical spectrum of the optical signal transmitted to the monitoring path so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS.
It is preferable that the optical signal transmitted to the monitoring path is detected by a photodetector to obtain its electrical spectrum.
It is also preferable that the compensation controller includes a comparing unit for comparing a presently detected electrical power filtered at the predetermined frequency satisfying the previously described principle of operation, with the previously detected one, and a feedback signal applying unit for applying a feedback signal to the PC so that the one having a smaller magnitude between the presently detected electrical signal and the previously detected electrical signal is selected as the result of comparison.
In order to achieve the first, second, and third objects, according to another aspect of the present invention, there is provided a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The method comprises (a) transforming states and directions of polarization components of an optical signal received from the optical transmission line, (b) rotating the polarization components of the optical signal output from the PC, (c) separating two orthogonal polarization components of the optical signal output from the polarization rotator so that a first polarization component is transmitted to an signal output path and a second polarization component is transmitted to a monitoring path, (d) controlling the PC using the electrical spectrum of the optical signal transmitted to the monitoring path so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS, and (e) controlling the polarization rotator by comparing an optical power of the first polarization component transmitted to the signal output path with an optical power of the second polarization component transmitted to the monitoring path.
It is preferable that step (d) comprises detecting the band-pass filtered electrical power at the predetermined frequency satisfying the previously described principle of operation, comparing a presently detected electrical power with the previously detected one, and applying a feedback signal to the PC so that the one having a smaller magnitude between the presently detected electrical signal and the previously detected electrical signal is selected as the result of comparison.
It is also preferable that step (e) comprises measuring optical powers of the first polarization component and the second polarization component, respectively, and comparing a first optical power of the first polarization component with a second optical power of the second polarization component, and applying a 90 degree rotation command to the polarization rotator in a case where the first optical power is smaller than the second optical power as the result of comparison.
In order to achieve the first, second, and third objects, according to another aspect of the present invention, there is provided a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The method comprises (a) exciting polarization of input light to optical transmission fiber with equal probability in all directions and transmitting the scrambled light to the transmission line, (b) transforming states and directions of polarization components of an optical signal received from the optical transmission line, (c) separating two orthogonal polarization components of the optical signal output from the PC so that a first polarization component is transmitted to an signal output path and a second polarization component is transmitted to a monitoring path, and (d) controlling the PC using the electrical spectrum of the optical signal transmitted to the monitoring path so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS.
It is preferable that step (d) comprises detecting the band-pass filtered electrical power at the predetermined frequency satisfying the previously described principle of operation, comparing a presently detected electrical power with the previously detected one, and applying a feedback signal to the PC so that the one having a smaller magnitude between the presently detected electrical signal and the previously detected electrical signal is selected as the result of comparison.
In order to achieve the first, second, and third objects, according to another aspect of the present invention, there is provided a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The method comprises (a) transforming states and directions of polarization components of an optical signal received from the optical transmission line, (b) rotating the polarization components of the optical signal output from the PC, (c) separating two orthogonal polarization components of the optical signal output from the polarization rotator so that a first polarization component is transmitted to an output path and a second polarization component is transmitted to a monitoring path, (d) detecting the band-pass filtered electrical power at the predetermined frequency satisfying the previously described principle of operation, (e) controlling the PC so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS until the detected electrical power is converged on a minimum value, and (f) controlling the polarization rotator so that a polarization component having a larger optical power between the first and second polarization components that are separated by the PBS is output, once the detected electrical power is converged on a minimum value.
In order to achieve the first, second, and third objects, according to another aspect of the present invention, there is provided a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber. The method comprises (a) exciting polarization of input light to optical transmission fiber with equal probability in all directions and transmitting the scrambled light to the transmission line, (b) transforming states and directions of polarization components of an optical signal received from the optical transmission line, (c) separating two orthogonal polarization components of the optical signal output from the PC so that a first polarization component is transmitted to an output path and a second polarization component is transmitted to a monitoring path, (d) detecting the band-pass filtered electrical power at the predetermined frequency satisfying the previously described principle of operation, and (e) controlling the PC so that the two orthogonal polarization components of the optical input signal to the PBS are aligned to two axes of the PBS until the detected electrical power is converged on a minimum value.